JK Rowling shocked and thrilled her fans in equal measure today, with the revelation that her new venture Pottermore was set to feature a wealth of new and previously unpublished material about the world of Harry Potter.

Although the author made clear that she had "no plans to write another novel", the fresh Potter material – to be unveiled later this year - already stretches to 18,000 words about the novels' characters, places and objects, with more to come. From Professor McGonagall's love for a Muggle as a young woman, to how the Dursleys met (Petunia was working in an office); from new information about Slytherin, Ravenclaw and Hufflepuff houses, to details about wand wood, Rowling's writing will be just one part of the richly interactive, free Pottermore.com website, which is intended to bring the Harry Potter storylines to interactive life for readers.

"I had more than half of the new material already written or in note form. I literally dug some out of boxes," said the author at a press conference this morning to announce the launch of the website, which she and the Pottermore management team have been working on for two years with UK digital agency TH_NK. "I generated more material than ever appeared in the books. I thought 'who would ever want to know the significance of all the difference wand woods?' ... Now you can go and see. It's such a rich experience to do it this way."

The material will be used on the new, free Pottermore website, a collaborative project for fans set in the Harry Potter universe. "I wanted to give something back to the fans that have followed Harry so devotedly over the years, and to bring the stories to a new generation," Rowling revealed. "I hope fans and those new to Harry will have as much fun helping to shape Pottermore as I have. Just as I have contributed to the website, everyone else will be able to join in by submitting their own comments, drawings and other content in a safe and friendly environment. Pottermore has been designed as a place to share the stories with your friends as you journey through the site." The website will open first to a million users who register first on 31 July – Harry's birthday. These users will help shape the site, with its full launch to all users to take place in October.

Pottermore will also sell the long-awaited ebook versions of the Harry Potter books directly to users from October, as well as digital audiobooks. "It is my view that you can't hold back progress. Ebooks are here to stay. Personally I love print and paper. [but] very very recently for the first time I downloaded an ebook and it is miraculous, for travel and for children. So I feel great about taking Harry potter into this new medium," Rowling said. "We knew there was a big demand for ebooks but if it was going to be done we wanted it to be more than that ... I wanted to pull it back to reading, to the literary experience, the story experience, and this is what emerged."

Although Rowling's publisher Bloomsbury will receive a share of revenues from the ebooks, the digital editions, which will be compatible with all devices, will only be sold from the Pottermore website, thus disintermediating other booksellers such as Amazon. "It means we can guarantee people everywhere are getting the same experience," said Rowling, of her decision to go it alone. "[I am] lucky to have the resources to do it myself and am therefore able to do it right. It's a fantastic and unique experience which I could afford in every sense. There was really no other way to do it."

Starting with the first novel in the series, Harry Potter and the Philosopher's Stone, Pottermore will allow its users to navigate their way through the story, with all-new illustrations and interactive "moments". Users start out by choosing a magical username, and as they move through the chapters of the book they will be sorted into houses – Rowling herself has written a "vast pool" of questions to direct users to their correct home – choose wands, shop on Diagon Alley and experience life at Hogwarts, just like Harry. Points can be won for houses by casting spells and mixing potions, users will be able to comment and add their own drawings and content – and Rowling herself will be dropping in "as a normal punter" now and then.

"If you are not sorted into Gryffindor, if you go into one of the other three houses, you will effectively get an extra quarter of a chapter. You will go off into your own common room, meet your own prefect, and find out what the true nature of the house is. In the main novel you only see the houses through the eyes of the heroes. So it's not a terrible thing to be in Slytherin," said the author, who admitted it was "a little frightening" how easily she slipped back into writing about Potter. "It's exactly like an ex-boyfriend ... I've never cried for a man as I cried for Harry Potter. Now we're casually dating and we have been for two years."

The author, who has sold 450m copies of the Harry Potter books worldwide, said that she still receives a "huge" amount of fan mail – "drawings, stories, ideas, suggestions I write prequels and sequels", so she felt the site was "a really great way to give back to the Harry Potter readership, who have obviously made the books such a huge success".

"This site is a fantastic way for fan creativity to continue. It's amazing for me to be creative in this medium, which didn't exist back in 1990," she added.

For the moment, Pottermore will be restricted to the world of the first book, Harry Potter and the Philosopher's Stone, but all seven books will be added in due course, complete with new material from Rowling – including, she promised, a more detailed explanation of Quidditch. "The number of geeky men who come up to me to argue about Quidditch – well, I'd be a lot richer if I got a quid for every one," she said. "They just think it's illogical. It's not. I had a speech by Dumbledore in the first book explaining why it's not illogical, but it never made it in. It will do at some point."